1. Field of the Invention
The present invention relates to a passive bending section for an endoscope which has a helical tube, and an endoscope having this passive bending section.
2. Description of the Related Art
When an inserting section of an endoscope is inserted into, for example, a lumen, the inserting section needs to smoothly bend from a distal end portion of the inserting section toward a proximal end portion of the inserting section to reduce a patient's pains and improve insertion-removal properties. Such a structure is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-218232 and Jpn. Pat. Appln. KOKAI Publication No. 2012-120573.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2006-218232 discloses a flexible tube for an endoscope. This flexible tube for the endoscope comprises a bending section, a first flexible tube connected to a proximal end of the bending section, and a second flexible tube connected to a proximal end of the first flexible tube. The first flexible tube has a helical tube, a reticular tube that covers an outer peripheral surface of the helical tube, and an envelope that is made of a resin and covers an outer peripheral surface of the reticular tube. The second flexible tube has a configuration similar to the first flexible tube. The first flexible tube and the second flexible tube are passively bent. A flexural rigidity increases from a distal end of the first flexible tube toward a proximal end of the second flexible tube.
The bending section is constituted by rotatably coupling node rings to one another. In particular, a first bending section is actively bent by a bending operation.
The first flexible tube is a curvature changing section and the second flexible tube is a force amount transfer section. The helical tube of the curvature changing section is integral with the helical tube of the force amount transfer section, and the reticular tube of the curvature changing section is integral with the reticular tube of the force amount transfer section. In the envelope of the curvature changing section, a flexibility of the envelope decreases from a distal end of the envelope toward a proximal end of the envelope. In the envelope of the force amount transfer section, the flexibility of the envelope is constant and equal to the flexibility of the proximal end of the envelope disposed in the curvature changing section.
When the bending section bends to the maximum, a maximum bend radius of the bending section is defined as R1. When a distal end portion of the curvature changing section bends to the maximum, a maximum bend radius of this distal end portion is defined as R2. In this case, R1>R2 is defined. The bend radius of the curvature changing section is gradually increased from a distal end of the curvature changing section toward a proximal end thereof by the abovementioned flexibility of the envelope. In a maximum bend state of the curvature changing section and a maximum bend state of the force amount transfer section, a curvature radius of the curvature changing section is smaller than a curvature radius of the force amount transfer section due to the abovementioned flexibility of the envelope.
When the inserting section bends to the maximum, a bend radius or a curvature of the inserting section changes at a substantially constant ratio due to the abovementioned flexibility of the envelope, and the inserting section smoothly bends from a distal end of the inserting section toward a proximal end thereof. In this way, the smoothness of the bending is acquired.
In general, the flexible tube has the flexibility, and hence the smoothness of the bending is acquired, but the bending is not regulated. Consequently, when the flexible tube abuts on, for example, a wall surface of a lumen, the flexible tube might infinitely be bent by an external force received from the wall surface. The term lumen refers to a region with complex bends as in, for example, a sigmoid colon in a large intestine. In a state where the flexible tube is bent, it is not easy to pass the inserting section including the flexible tube through the bent region. When the flexible tube is inserted into and removed from the lumen, the flexible tube needs to push back the lumen against the force applied from the lumen to the flexible tube.
Thus, for example, in Jpn. Pat. Appln. KOKAI Publication No. 2012-120573, a flexible tube having a helical tube, a reticular tube and an envelope is disclosed. This helical tube has a densely coil and an initial tension is given to the whole densely coil. The flexible tube is not easily bent by the initial tension, and acquires elasticity to push back the lumen. The elasticity have an influence on insertion-removal properties of the flexible tube in a body cavity (in a lumen) of the large intestine or the like. The elasticity include, for example, bouncing properties, a repulsive elasticity, hysteresis, spring properties, a resilience, a flexural rigidity and the like, and have properties to return the bent flexible tube back to a substantially straight state.